Semifinished products, particularly tubes made of cross-linked polyolefins are obtained using, for example, an extrusion method, see German Pat. No. 1,679,826 and European Pat. No. 0,003,587. Processing of such material is relatively difficult due to its high stiffness. Thus it has been proposed, see, for example, European Patent Application No. 82,100,466.0, to shape semifinished products made of tubes by stretching the tube during its production and subsequently heating the tube to a temperature above the crystallite melting point for the shaping operation. During this operation the tube re-assumesits original nonstretched form and thereby is able to adapt to suitable respectively presented molds. Since the cross-linked material is difficult to process there existed hitherto the tendency of carrying out the cross-linking during or after shaping. Consequently, the advantageous properties of the cross-linked material could only be utilized for the limited number of products which could be produced in this manner.